Lithium ion secondary batteries are small and light, high in energy density, and capable of repeated charging and discharging. For environmental reasons as well, the demand for such lithium ion secondary batteries is expected to increase. These high energy-density lithium ion secondary batteries are used in fields such as mobile phones and notebook computers, but with the expansion and development of applications for these batteries, even better performance is required, such as lower resistance and higher capacity.
A separator fulfills an important function for preventing an electric short circuit between the positive electrode and the negative electrode of a lithium ion secondary battery. A fine porous membrane made of polyolefinic resin, for example, is typically used as the separator of a lithium ion secondary battery. The separator also normally has the role of maintaining the safety of the lithium ion secondary battery by fusing when the battery reaches a high internal temperature near 130° C., for example, and blocking the fine pores to prevent transfer of lithium ions, thereby achieving a shutdown function to shut off the current. If, however, instantaneous heat generation causes the battery temperature to further exceed the melting point of the resin forming the separator, then the separator may suddenly contract, and a location where the positive electrode and negative electrode come in direct contact and short-circuit may expand. In this case, the battery temperature may rise to several hundred ° C. or higher, and the battery may reach a state of abnormal overheating.
To inhibit a temperature increase during abnormal overheating, use of heat expandable microcapsules has been proposed. Patent literature (PTL) 1 discloses a current collector that includes a resin layer and a conductive adhesive layer. Heat expandable microcapsules formed by enclosing expandable material in a shell that contains a thermoplastic polymer material are used in the adhesive layer. PTL 2 discloses dispersing, in an electrode active material layer, heat expandable microcapsules formed by enclosing a low-boiling hydrocarbon in a copolymer shell. PTL 3 discloses adding, to an electrolysis solution, heat expandable microcapsules in which a low-boiling hydrocarbon or a foaming agent is polymerized in situ in a thermoplastic resin, such as vinylidene chloride or acrylonitrile.